The research objectives of this K08 career development award are to to define shared patterns of hepatitis C virus (HCV) envelope mutation that mediate escape from autologous neutralizing antibody (nAb), to measure the fitness costs of those mutations, and to define the human B cell repertoire encoding broadly neutralizing antibodies induced during HCV infection. Through mentoring, structured didactics and laboratory experience, this K08 career development award is the ideal mechanism to allow Dr. Justin Bailey to become an independent investigator conducting translational research on broadly neutralizing antibody responses in HCV infection. Candidate: Dr. Bailey is a physician in his final year of fellowship training in Infectious Diseases at the Johns Hopkins Hospital. He earned an M.D. and Ph.D. in Immunology at the Johns Hopkins University School of Medicine through the Medical Scientist Training Program in 2007 and completed residency training in Internal Medicine at the Massachusetts General Hospital through the ABIM research pathway in 2009. He has spent the research years of his Infectious Diseases fellowship studying hepatitis C virus (HCV) envelope evolution with his primary mentor, Dr. Stuart Ray. His short term goals are to identify and characterize HCV nAb escape mutations, to define genetic characteristics of a broad nAb response, to gain practical experience in hybridoma generation and deep sequencing, and to pursue didactic training in bioinformatic methods and B cell immunology. His long term goal is to become an independent investigator studying the mechanisms by which viral evolution and evolution of the B cell repertoire can lead to a broad nAb response. Environment: The investigations described in this proposal will be performed in the laboratories of Dr Stuart Ray, a Professor in the Infectious Diseases Division at Johns Hopkins Hospital and an internationally- recognized expert in HCV virology, humoral immunity, and computational biology and Dr. James E. Crowe, Jr., a Professor of Pediatrics, Microbiology and Immunology at Vanderbilt University Medical Center and an expert in B cell biology and the molecular basis for the development of effective B cell responses to viruses in humans. Both mentors are leaders in their respective fields, are well-funded, and have a strong track record of mentoring young investigators to independence. Necessary equipment is available in the labs of both mentors for the completion of the proposed studies, and relevant patient samples, antibodies, and other reagents are readily available. Research: A vaccine against hepatitis C virus (HCV) is urgently needed, and induction of an effective neutralizing antibody response will likely be a keystone of any successful vaccine strategy. The difficulty lies in the fact that HCV replicates to high levels using an error-prone polymerase, leading to extensive world-wide genetic diversity of the virus and rapid viral evolution in infected individuals. Most antibodies induced during natural infection do not have broad enough specificity to neutralize diverse HCV isolates, and viral evolution can lead to escape from nAb responses in individuals who become infected. However, rare infected individuals develop broad nAb responses, and nAb escape mutations in some epitopes may convey significant fitness cost to the virus. The goal of this investigation is to define shared patterns of hepatitis C virus (HCV) envelope mutation that mediate escape from autologous neutralizing antibody (nAb), to measure the fitness costs of those mutations, and to define the human B cell repertoire encoding broadly neutralizing antibodies induced during HCV infection. A better understanding of these questions will facilitate HCV vaccine development by identifying immunogenic but fitness-constrained epitopes that might serve as vaccine antigens and also by defining genetic features of broadly nAb that should be induced by a vaccine against HCV. The research described in this proposal is novel, achievable within the award period, and of high potential significance. Through the research experience acquired through completion of this work, intensive mentoring, and structured didactics, this K08 career development award will allow the primary investigator, Dr. Justin Bailey, to become an independent investigator conducting translational research on broadly neutralizing antibody responses in HCV infection. Relevance The World Health Organization estimates there are 170 million persons with hepatitis C virus (HCV) infection worldwide, and in most countries, HCV infection is found in 1-2% of the general population. A vaccine to prevent or attenuate HCV infection is urgently needed, and stimulation of effective neutralizing antibody (nAb) responses will likely be a keystone of any successful vaccine strategy. Therefore, a better understanding of viral escape from nAb and development of broadly nAb responses should facilitate HCV vaccine development.